Actinic moth trap
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My initial reaction to the photo is that it is Double Square-spot, but I'm having a hard job saying why I think that, other than going by instinct (always a dangerous thing to do!). In my experience Double Square-spot is seen far more often than Triple-spotted Clay, but then that is what the result would be if my instinct always told me that moths looking like this were Double Square-spot ...
Will be interesting to see what others think.
Entomologist and biological recorder
I'll wave the white flag on this. There was a long discussion about the 2 species on the Yahoo ukmoths group last year and it seems that the hindwing colour in the real clue. Even this, however, is not clear cut. Read the description in Waring and Townsend (1st edition) and you are not a lot wiser; look at the pictures in Skinner and seems to be glaringly obvious.
This is, perhaps, one of those species pairs that you get to know at your own site, but given a picture from elsewhere, you get completely thrown.
I'm with the majority here - I'd put it as Double Square-Spot, as when I trap them the TSCs seem to be far more drab than the well-marked DSSs, but they seem to vary in tandem across the country.
Record your ladybird sightings!
I had a look at a whole caught series of both species side by side recently and once you see them together there are a few things that stand out. The hindwings of TSC are very pale compared to the forewings, in DSS there is less contrast. also the forwings of TSC when in comparison with DSS appear longer and slimmer. Also TSC tends to have a more purplish colour to the forewings but that can be rather subjective. Having said that as Robert says its all much easier in comparison that with one specimen on its own from a photo. howere we wouuld not accept a record in our county where we had not seen at least a good photo showing the fore and hindwings. I should add that I would say this is DSS.
Joint Macro-moth recorder for Bedfordshire, VC30
Lat/Lng: 51.5, -0.4
OS grid ref: TQ1689